(Chem.) To clean (a filter, a chromatography column, a water softener, etc.) by passing liquid through in the reverse of the normal direction of flow. This procedure dislodges particles which clog the pores of the filter or column packing, removing them from the filter or column.

... is but the backwash of a wave... Well, then, I went up the open way, and came in a few miles of that hot afternoon to the second ridge of the Jura, which they call 'the Terrible Hill', or 'the Mount Terrible'—and, ...— The Path to Rome • Hilaire Belloc

... ten years after Waterloo, while the country was still in the backwash of the long-drawn Napoleonic wars. It was a time of material reconstruction and expansion, while social reconstruction lagged sadly and angrily behind. The year of his birth saw the first railway opened in England; it ...— Thomas Henry Huxley - A Character Sketch • Leonard Huxley